Reactive phenolic resins in the preparation of highly viscous self-adhesive compositions

ABSTRACT

Hot-melt pressure-sensitive adhesive based one or more non-thermoplastic elastomers, at least comprising 100 parts by mass of the non-thermoplastic elastomer(s), from 1 to 200 parts by mass of one or more tackifying resins, from 1 to 100 parts by mass of one or more reactive phenolic resins whose methylol content is from 1 to 20% by weight, based on the reactive phenolic resin, from 1 to 100 parts by mass of accelerator substances.

The present invention relates to the use of reactive phenolic resins forthe chemical/thermal crosslinking of hot-melt pressure-sensitiveadhesives (PSAs) based on non-thermoplastic elastomers, such as naturalrubber, for example, using tackifying resins, optionally fillers andplasticizers, and to the application of these hot-melt PSAs to produceself-adhesive articles, especially for producing high-performanceself-adhesive articles such as tapes or labels.

From the conventional technology of the solvent-based preparation andcoating, for example, of natural-rubber pressure-sensitive adhesives,the use of reactive phenolic resins for crosslinking the adhesivecompositions is known.

Described as suitable for accelerating this reaction are organometallicderivatives of metals of group IV of the Periodic Table, inorganicactivators (zinc oxide, zinc resinates), and acids.

Other vulcanization accelerators are sidechain-halogenated reactivephenolic resins, polychloroprenes, chlorinated paraffins, zinc stearatesand metal chlorides such as, for example, Zn(II) chloride, SnCl₂*2H₂O,FeCl₃*6H₂O.

In hot-melt pressure-sensitive adhesives based on non-thermoplasticelastomers, however, the use of the known thermal crosslinking systemshas not been possible to date, since process problems occur in thecourse of the preparation.

This prior art is depicted at length, for example, in “Donatas Satas,Handbook of Pressure Sensitive Adhesive Technology”, Second Edition, NewYork, 1989, p. 363, or Werner Hofmann, “Vulkanisation undVulkanisationshilfsmittel” [Vulcanization and Vulcanizing Auxiliaries],1965, BAYER.

The hot-melt PSAs developed in recent years on the polymer basis ofnon-thermoplastic elastomers, such as, for example, natural rubber orother high molecular mass rubbers, in the absence of a crosslinking stepafter application lack sufficient cohesion for the majority ofapplications. This is manifested in inadequate shear strength of theself-adhesive tapes manufactured in this way and may even lead to theformation of disruptive residues of adhesive, which make it impossibleto achieve a desired residueless redetachability after use.

For many years, this deficiency prevented the use of hot-melt PSAs basedon natural rubber in the self-adhesive tape applications traditionallydominated heavily by natural rubber, such as masking tapes or adhesivetapes for packaging.

The crosslinking processes used to date for hot-melt PSAs based onnon-thermoplastic elastomers, by means of ionizing radiation (electronbeams=EBC or ultraviolet light=UV), require the presence of appropriate,cost-intensive installations such as radiation sources and complexprotective equipment, especially at relatively high film thicknesses.

Furthermore, in the case of many customary ingredients such as fillers,non-transparent resins and pigments, and in the case of thick films ofadhesive, UV crosslinking is possible only to an extremely limitedextent.

The use of exclusively non-thermoplastic rubbers as an elastomercomponent in the formulation of PSAs with the existing cost advantagepossessed, for example, by natural rubbers over the standard commercialblock copolymers, and the outstanding properties, especially the shearstrength of natural rubber and of corresponding synthetic rubbers, andalso processes for preparing, applying and crosslinking hot-melt PSAsbased on non-thermoplastic elastomers, are also set out at length in thepatents WO 9411175 A1, WO 9525774 A1, WO 9707963 A1 and,correspondingly, U.S. Pat. No. 5,539,033, U.S. Pat. No. 5,550,175, andalso EP 0 751 980 B1 and EP 0 668 819 B1.

In these cases, the additives customary in PSA technology, such astackifier resins, plasticizers and fillers, are described.

The preparation process disclosed in each case is based on a twin-screwextruder which permits compounding to a homogeneous PSA blend with thechosen process regime, involving mastication of the rubber andsubsequent gradual addition of the individual additives with anappropriate temperature regime.

The mastication step of the rubber, which precedes the actual productionprocess, is described at length. It is necessary and characteristic ofthe process chosen, since with the technology selected therein it isindispensable to the subsequent integration of the other components andto the extrudability of the blended hot-melt pressure-sensitiveadhesive. Also described is the feeding-in of atmospheric oxygen, asrecommended by R. Brzoskowski, J. L. and B. Kalvani in Kunststoffe 80(8), (1990), p. 922 ff., in order to accelerate mastication of therubber.

This procedure makes it absolutely necessary to practise the subsequentstep of electron beam crosslinking (EBC), and to use reactive substancesas EBC promoters in order to achieve an effective crosslinking yield.

Both process steps are described in the abovementioned patents, but theEBC promoters chosen also tend towards unwanted chemical crosslinkingreactions at elevated temperatures, which limits the use of certaintackifying resins.

Owing to the unavoidable high product temperatures, compounding in atwin-screw extruder prevents the use of heat-activatable substancessuitable for crosslinking the self-adhesive compositions, such as, forexample, reactive (optionally halogenated) phenolic resins, sulphur orsulphur-donor crosslinker systems, since the chemical crosslinkingreactions which ensue in the process result in such a great increase inviscosity that the coatability of the resulting hot-meltpressure-sensitive adhesive is impaired.

The patent application JP 95 278 509 discloses a self-adhesive tape inwhose production natural rubber is masticated to an average molecularweight M_(w)=100,000 to 500,000 in order to obtain a coatablehomogeneous mixture comprising hydrocarbon resins,rosin/rosin-derivative resins or terpene resins, which can be processedregularly at between 140° C. and 200° C. with a coating viscosity offrom 10 to 50×10³ cps, but subsequently requires an extremely high EBCdose (40 Mrad) in order to ensure the shear strength necessary for itsuse.

For backing materials such as impregnated and/or sized papers, and forwoven backings based on viscose staple and the like, the system is notvery suitable, since at the necessarily high beam doses there issignificant deterioration of the backing.

A disadvantage of the crosslinking technologies (essentially EBCirradiation) described in the documents cited, in addition to thecapital investment required, is the damage of certain sensitive backingsby electron beams. This is manifested to a particular extent in the caseof paper backings, viscose staple wovens, and siliconized releasepapers, but especially in the case of widespread film materials such aspolypropylene, by a deterioration in the elongation-at-break properties.

Moreover, many standard commercial PVC films tend to discolour under EBCirradiation, such discoloration having a deleterious effect in the caseof light-coloured or transparent film grades.

Furthermore, many of the release coatings which are customary inadhesive tape manufacture are damaged by electron beam irradiation andso are impaired in their effect. In an extreme case, this may result inthe non-unrollability of adhesive tape rolls or in the non-reusabilityof transfer release papers, which are required in the adhesive tapeproduction process.

Certain synthetic rubbers such as polyisobutylene (PIB), butyl rubber(IIR) and halogenated butyl rubber (XIIR), finally, are not amenable toelectron crosslinking and are degraded under irradiation.

One way of minimizing these disadvantages consists in the use of certainsubstances which lessen the required beam dose and thus the concomitantdamage. A range of such substances are known for use as EBC promoters.However, EBC promoters may also tend towards unwanted chemicalcrosslinking reactions at elevated temperatures, which limits theselection of the EBC promoters that can be used for hot-melt PSAproduction and, moreover, restricts the use of certain tackifyingresins. These restrictions, and certain advantageous combinations of EBCpromoters and non-crosslinking phenolic tackifier resins, are thesubject-matter, in particular, of the document WO 97/07963.

The use of non-thermoplastic elastomers is also described in JP 95 331197, where use is made of natural rubber having an average molecularweight (weight average) M_(w)<1 million g/mol with aliphatic,non-reactive hydrocarbon resins, which is blended with blockedisocyanates, precrosslinked at 150° C. for five minutes, and, followingits subsequent coating onto PET film, is cured at 180° C. for severalminutes (for example 15 minutes).

A disadvantage of this process, firstly, is the blocking agent releasedduring the crosslinking reaction, which on the one hand, if it remainsin the hot-melt pressure-sensitive adhesive, may impair the adhesionproperties of the tape in a variety of respects, and on the other hand,on its escape in vapour form, leads to coating defects such asporosities and necessitates complex technology in order to draw offthese blocking agents under suction and remove them.

Of particular disadvantage, however, is the high crosslinkingtemperature, which rules out temperature-sensitive backings such as manyfilms and foams on principle and in the case of paper backings andrelease papers may lead to embrittlement.

In summary it may be stated that crosslinking of the known hot-meltpressure-sensitive adhesives based on non-thermoplastic elastomersnecessitates either damagingly high radiation doses or else damaginglyhigh temperatures with long crosslinking times, and both have theconsequence of damage in the case of a large number of the customarybacking materials.

The object of the invention is to remedy this situation and to combinethe economic advantages of the solvent-free manufacture and applicationof hot-melt pressure-sensitive adhesives based on non-thermoplasticelastomers with the chemical-thermal crosslinking possibilities of theconventional solvent technology for thick adhesive compositions,including filled and coloured hot-melt pressure-sensitive adhesives,with high film thicknesses on radiation- and temperature-sensitivebacking materials and to remedy the disadvantages of thermalcrosslinkers which can be used in hot-melt pressure-sensitive adhesivesaccording to the prior art.

This object is achieved by means of a hot-melt pressure-sensitiveadhesive as characterized more closely in the main claim. The subsidiaryclaims relate to advantageous developments of the subject-matter of theinvention, to advantageous possibilities for use, and to processes forproducing the backing materials coated with the subject-matter of theinvention.

The invention accordingly provides a hot-melt pressure-sensitiveadhesive (PSA) based on one or more non-thermoplastic elastomers whichcomprises:

-   -   100 parts by mass of the non-thermoplastic elastomer(s),    -   from 1 to 200 parts by mass of one or more tackifying resins,    -   from 1 to 100 parts by mass of one or more reactive, optionally        also halogenated, phenol-formaldehyde resins whose methylol        content is between 1 and 20% by weight and/or whose halogen        content is between 1 and 20% by weight, based in each case on        the reactive phenolic resin,    -   from 1 to 100 parts by mass of accelerator substances.

The crosslinking of hot-melt pressure-sensitive adhesives based onnon-thermoplastic elastomers with phenol-formaldehyde resins has aparticular advantage in that a desired residue-free redetachabilityafter use is made possible.

Preferably, the elastomer or elastomer mixture has an average molar massof from 300,000 to 1.5*10⁶ g/mol, determined as the weight average usinga GPC measurement. In the GPC measurement (gel permeationchromatography, a liquid chromatography conducted in the form of acolumn chromatography) a liquid phase comprising the dissolved polymeris passed through a gel. Smaller molecules of the dissolved substanceare able to penetrate (diffuse) into all the pores; to them, the entirevolume of the mobile phase in the separating column is available. Forthis reason, they are retained longer in the column than are the largermolecules. These molecules, which are larger than the largest pores ofthe swollen gel, are unable to penetrate the gel particles and migratepast them; they leave the column first. Consequently, in the eluate, themolecules appear in the order of decreasing molecular size. Since themolecular size is generally proportional to the molar mass, gelchromatography offers the possibility of separating and purifyingsubstances of different molar masses and of determining molar masses.

With further preference, the hot-melt PSA in the uncrosslinked state hasa complex viscosity of from 10,000 to 300,000 Pa*s at 0.1 rad/s and 110°C., preferably from 30,000 to 170,000 Pa*s at 0.1 rad/s and 110° C.,very particularly from 40,000 to 140,000 Pa*s at 0.1 rad/s and 110° C.

The non-thermoplastic elastomers are advantageously selected from thefollowing group, either individually or in any desired mixture:

-   -   natural rubbers    -   random-copolymerized styrene-butadiene rubbers (SBR)    -   butadiene rubbers (BR)    -   synthetic polyisoprenes (IR)    -   butyl rubbers (IIR)    -   halogenated butyl rubbers (XIIR)    -   ethylene-vinyl acetate copolymers (EVA).

In a further advantageous development of the invention, the hot-melt PSAcomprises a polymer blend of one or more of the non-thermoplasticelastomers and one or more thermoplastic elastomers, the latter againbeing selectable from the subsequent listing, individually or in anydesired mixture:

-   -   polypropylenes    -   polyethylenes    -   metallocene-catalysed polyolefins    -   polyesters    -   polystyrenes    -   synthetic block copolymer rubbers

As reactive phenolic resins it is also possible to use mixtures ofreactive phenolic resins which are distinguished by differingreactivity. It is further possible to use reactive, halogenated phenolicresins which are notable for increased reactivity.

Reactive phenolic resins which may be used include the following, in alist which should not be understood as being conclusive.

Manufacturer: SCHENECTADY EUROPE S. A., Béthune, France DesignationMethylol content, % by weight SP 103 12 SFP 121 17 SP 126 11 SP 134 16SP 154 10 FRJ 551 14 SFP 183 15

Manufacturer: VIANOVA Resins GmbH, Wiesbaden, Germany DesignationMethylol content, % by weight Vulkaresen PA 510 6-9 Vulkaresen PA 13011-14Manufacturer: SCHENECTADY EUROPE S. A., Béthune, France

(reactive, brominated phenolic resins) Methylol content, Brominecontent, Designation % by weight % by weight SP 1055 10-14 3.5-4.5 SP1056 7.5-11  6-9

The crosslinking reaction may take place in the usual manner on the openbelt in tunnel installations with an appropriate temperature regime.

For the crosslinking of the hot-melt PSAs it is also possible to utilizethe heat treatment processes which are often used in adhesive tapeproduction and are as required, for example, for the relaxation of filmmaterials, or crosslinking may be effected at room temperature on thebelt.

The accelerator substances are chosen in particular from the group oforganic acids, especially resins containing acid groups, metal oxides,metal stearates, metal resinates, chlorinated paraffins, chloroprenes,chlorinated and brominated butyl rubbers, or chlorosulphonatedpolyethylenes.

To the hot-melt PSA it is possible to add fillers, which may inparticular be selected from the group consisting of metal oxides,chalks, with particular preference chalks having specific surface areasof from 3 to 20 m²/g, precipitated or pyrogenic silicas, with particularpreference silicas having specific surface areas of from 20 to 250 m²/g,preferably from 40 to 200 m²/g, solid or hollow glass beads, withparticular preference the solid or hollow glass beads having an averagediameter of from 3 to 200 μm, preferably from 5 to 135 μm,microballoons, carbon blacks, with particular preference carbon blackshaving specific surface areas of from 20 to 120 m²/g, and/or glassfibres or polymer fibres.

It is also possible for the surface-modified variants of the fillersrecited above to find application.

The microballoons are elastic, thermoplastic hollow beads which have apolymer shell. These beads are filled with low-boiling liquids or withliquefied gas. Suitable shell polymers are, in particular,acrylonitrile, PVDC, PVC or acrylates. Hydrocarbons such as the loweralkanes, pentane, for example, are suitable as the low-boiling liquid,while a suitable liquefied gas is a chemical such as isobutane.

Particularly advantageous properties are manifested when themicroballoons have a diameter at 25° C. of from 3 μm to 40 μm, inparticular from 5 μm to 20 μm.

By exposure to heat, the capsules expand irreversibly andthree-dimensionally. Expansion comes to an end when the internalpressure is equal to the external pressure. In this way, a closed-cellfoam backing is obtained which features good flow behaviour and highrecovery forces.

Following thermal expansion due to elevated temperature, themicroballoons advantageously have a diameter of from 20 μm to 200 μm, inparticular from 40 μm to 100 μm.

This expansion may take place prior to or following the incorporationinto the polymer matrix, or else before or after incorporation into thepolymer matrix and shaping.

It is also possible to perform the expansion following incorporationinto the polymer matrix and prior to shaping.

The fillers should be added in proportions of from 1 to 300 parts bymass per 100 parts of elastomer, individually or in any desiredcombination of the individual fillers.

It is further advantageous to admix plasticizers into the hot-melt PSA,the said plasticizers in turn being selected in particular from thegroup consisting of paraffinic or naphthenic oils, with particularpreference paraffinic or naphthenic oils having kinematic viscosities at20° C. of between 40 and 255 mm²/s, oligomeric nitrile rubbers, withparticular preference liquid nitrile rubbers having ACN contents of from20 to 40% by weight, in particular from 20 to 35% by weight, liquidisoprene rubbers, with particular preference isoprene rubbers havingmolar masses of between 10,000 and 70,000 g/mol, oligobutadienes, withparticular preference oligobutadienes or functionalized oligobutadieneshaving molar masses of from 1500 to 70,000 g/mol, soft resins, withparticular preference soft resins having molar masses of from 100 to2000 g/mol, in particular from 250 to 1700 g/mol, wool fats and/orrapeseed oils and castor oils.

The hot-melt PSA of the invention may find application in the productionof a self-adhesive article by application of the said adhesive to atleast one side of a web-form material, for example a material coatedanti-adhesively on both sides, the hot-melt PSA being applied with apreferred rate of from 5 to 3000 g/m², with particular preference from10 to 200 g/m².

The web-form material may in particular comprise a single-sidedly ordouble-sidedly coated paper backing or a single-sidedly ordouble-sidedly coated polymer film backing, in which case theapplication rate may be from 5 to 200 g/m² and in particular from 10 to100 g/m².

As backings it is further possible to use wovens or nonwovens of allkinds.

Consolidated nonwoven webs are produced, for example, on stichbondingmachines of the “Malifleece” type from the company Malimo and can beobtained, inter alia, from the companies Naue Fasertechnik and TechtexGmbH. A Malifleece is characterized in that a cross-laid web isconsolidated by the formation of loops from fibres of the web.

The backing used may also be a web of the Kunit or Multiknit type. AKunit web is characterized in that it originates from the processing ofa longitudinally oriented fibre web to form a sheetlike structure whichhas the heads and legs of loops on one side and, on the other, loop feetor pile fibre folds, but possesses neither filaments nor prefabricatedsheet-structures. A web of this kind has also been produced for manyyears, for example, on stitchbonding machines of the “Kunitvlies” typefrom the company Karl Mayer, formerly Malimo. A further characterizingfeature of this web is that, as a longitudinal-fibre web, it is able toabsorb high tensile forces in the lengthwise direction. Thecharacteristic feature of a Multiknit web relative to the Kunit is thatthe web is consolidated on both the top and bottom sides by virtue ofthe double-sided needle punching.

Finally, stitchbonded webs are also suitable. A stitchbonded web isformed from a nonwoven material having a large number of stitchesextending parallel to one another. These stitches are formed by theincorporation, by stitching or knitting, of textile filaments. For thistype of web, stitchbonding machines of the type “Maliwatt” from thecompany Karl Mayer, formerly Malimo, are known.

Starting materials envisaged for the textile backing are, in particular,polyester fibres, polypropylene fibres or cotton fibres. The presentinvention, however, is not restricted to the aforementioned materials;rather, a large number of other fibres may be used to produce the web.

Needlepunched, wet-laid and/or air-jet- and/or water-jet-consolidatedwebs may be obtained, for example, from the company Freudenberg.

Particularly suitable thicknesses which have been found for the hot-meltPSA on the web-form material are between 5 μm and 3000 μm, preferablybetween 15 μm and 150 μm.

Furthermore, the hot-melt PSA may have been applied to a double-sidedlyantiadhesively coated release paper in a thickness of from 20 μm to 3000μm, in particular from 40 μm to 1500 μm.

One particularly suitable process for producing the self-adhesivearticles set out above, especially for producing high-performanceself-adhesive articles such as tapes or labels, is to apply the hot-meltPSA using a multi-roll applicator unit comprising from two to fiverolls.

EXAMPLES

The examples which follow are intended to illustrate the use, inaccordance with the invention, of reactive phenol-formaldehyde resinswhich are accelerated in their crosslinker action and are activated byadditional substances for the chemical crosslinking of natural-rubberhot-melt pressure-sensitive adhesives, without restricting theinvention.

The test methods used are briefly characterized below:

The adhesion tests on the adhesive tape specimens were conducted in eachcase after a 24 h storage period at room temperature and, forcomparison, depending on example, after 7-day thermal conditioning at70° C., comparability of the results being ensured by the additionalstorage of the thermally conditioned specimens, prior to themeasurements, at 23° C. and 50% atmospheric humidity for 24 h.

Test Method 1: Bond Strength

The bond strength (peel strength) of the adhesive tape specimens wasdetermined in accordance with AFERA 4001.

Falling bond strength levels are generally a sign of increasing degreeof crosslinking of the hot-melt PSA.

Test Method 2: Shear Strength

The shear strength of the adhesive tape specimens examined wasdetermined in accordance with PSTC 7 (Holding Power). All valuesreported were determined at room temperature under the stated load of 20N with a bond area of 20×13 mm². The results are recorded in minutes ofholding time.

An increase in the shear stabilities within the range examined, for agiven hot-melt PSA formulation, denotes a higher degree of crosslinkingand/or higher cohesion.

In the wider sense it is also necessary to consider the type of failure,in connection with which the following applies:

-   -   undercrosslinked (weakly cohesive) hot-melt PSAs:        -   short shear stability times with cohesive failure    -   optimally crosslinked hot-melt PSAs:        -   long shear stability time    -   overcrosslinked (too cohesive) hot-melt PSAs:        -   short shear stability times as a result of adhesive failure            Test Method 3: Gel Content

The degree of crosslinking of the already applied natural-rubberhot-melt PSA was determined on the finished adhesive tape by way of thegel content of the hot-melt PSA. For this purpose, adhesive tape samplesin sections of 20 cm² were punched out and these sections were weldedinto a pouch made from polyethylene spunbonded fabric (commercial Tyvekfrom the company Du Pont, with a basis weight of approximately 55g/cm²). The specimens were extracted by shaking with toluene at roomtemperature for three days. The toluene was changed each day. Followingextraction, the toluene was replaced by hexane/heptane and the sampleswere dried at 110° C. The gel content was determined by differentialweighing, taking into account the extraction losses from the spunbondedfabric and from the backing.

The result is stated as the gel value, in percent, the initiallyuncrosslinked elastomers being taken as 100%.

Test method 4: Swelling Test

In a simplified procedure, it is also possible to determine the degreeof crosslinking of the hot-melt PSA on the polymer basis ofnon-thermoplastic elastomers in a comparative manner, from swellingmeasurements.

For this purpose, an adhesive tape strip is placed in specialboiling-point spirits 60/95 and then examined, visually and mechanicallyusing a spatula, for the presence and consistency of swollen hot-meltPSA gel remaining on the tape.

The result is stated as the “swelling test” and embraces a scale from 0to 6.

The ratings on this scale have the following meanings: Swelling Gelcontent test by method 4 rating Gel consistency in swelling test [%] 0Layer of composition is fragmentary and 0-5 sludgy, i.e. no crosslinkingevident 1 Severe swelling, composition very slimy and  5-15 mobile, i.e.very low crosslinking 2 Severe swelling, composition is slimy and 15-25easy to displace 3 Good swelling, composition less slimy and 25-35displaceable 4 Slight swelling, composition barely slimy, still 35-45displaceable 5 Virtually no swelling, composition almost a 45-55coherent layer and virtually impossible to displace 6 No swelling,composition forms a coherent >55 layer and can only be removed byscratching

The optimum balance between cohesion and adhesion, expressed by theswelling test rating of the PSA in question, depends on the applicationof the specific adhesive tape. For general-purpose masking tapes, theoptimum swelling test rating, for example, is 2-3; for high-temperaturemasking tapes with temperature stabilities of greater than 140° C. theoptimum swelling test rating is 4-5.

Test Method 5: Viscoelastic Properties of Hot-Melt PSAs

Finally, the degree of crosslinking of a given hot-melt PSA may bedetermined very simply from the measurement of its viscoelasticproperties. The evaluation of the results of these measurements requiresin each case comparison with the uncrosslinked state of the hot-meltPSA, since the formulation in this case has a very strong influence onthe absolute values measured. As a measure of crosslinking it ispossible to state both the ratio of the viscosity of the crosslinkedhot-melt PSA to the viscosity of the uncrosslinked hot-melt PSA and thecorresponding ratio of the loss angles, normally expressed as tan δ.

In order to determine the viscoelastic properties of hot-melt PSAs,dynamic-mechanical measurements were conducted in torsion rheometers,with oscillating deformations being predetermined and resulting shearstresses measured (in reference thereto see, for example, W. M. Kulicke“Flow behaviour of substances and mixtures of substances”, Hüthig andWepf, 1986).

In the examples, use was made of an instrument of the type RDA II(Rheometric Dynamic Analyzer II from Rheometric Scientific GmbH, atorsion rheometer with a plate/plate measuring system). A prepared,planar, bubble-free sample of the hot-melt PSA with a film thickness of1.5 mm was introduced into the measuring system. Measurement was in thetemperature range from −50° to +200° C. and in a frequency range of from0.1 rad/s to 100 rad/s with a constant standard force of 150 g.

The measurements were detected with computer assistance; theviscoelastic properties (storage modulus G′, loss modulus G″, loss angletan δ, complex viscosity η*) of the hot-melt PSA were determined for thestated temperature and frequency ranges in the customary manner from theextent and course of the measured shear stresses over time.

Measurements of this kind can be conducted both on prepared hot-melt PSAsamples and on complete adhesive tapes, it being necessary in the lattercase to laminate adhesive tape samples to one another until theappropriate film thickness is reached and to fasten the backing side ofthe topmost layer to the corresponding plate of the measuring systemusing an appropriate structural adhesive.

Test Method 6: Processing Viscosity of the Hot-Melt PSA

In order to obtain rapid information on the viscoelastic properties of ahot-melt PSA, especially the degree of degradation or, respectively,degree of crosslinking of the framework polymer, during production,dynamic-mechanical measurements in accordance with above-described testmethod 5 were conducted in the frequency range from 0.1 rad/s to 100rad/s at a constant standard force of 150 g, but with the measurementtemperature left constant at 110° C. Prior to measurement, the hot-meltPSA samples were preheated at 110° C. in the measuring system for 7minutes.

The value of the complex viscosity at a frequency of 0.1 rad/s and atemperature of 110° C. gives information, for identical formulation, onthe cohesiveness and, respectively, degree of crosslinking of thehot-melt PSA.

To quantify the degree of crosslinking, the crosslinking number, CN, isintroduced as the ratio of the respective complex viscosity of thecrosslinked hot-melt PSA formulation to the complex viscosity of theuncrosslinked hot-melt PSA formulation:

CN Viscosity of the crosslinked hot-melt PSA/viscosity of theuncrosslinked hot-melt PSA.

Test Method 7: Molar Mass Determination by Means of Gel PermeationChromatography

The molar masses of the elastomer fraction present in the natural-rubberhot-melt PSAs were determined exclusively on uncrosslinked hot-melt PSAsamples by means of GPC on polystyrene standard with the followingmeasurement system: Eluent: tetrahydrofuran (THF), analytical gradeColumns: PSS-SDV, 5 μm 103 Å, ID 8.0 mm × 300 mm PSS-SDV, 5 μm 105 Å, ID8.0 mm × 300 mm Precolumn: PSS-SDV, 5 μm 106 Å, ID 8.0 mm × 300 mm Pump:TSP P200 Flow rate: 1.0 ml/min Injection system: TSP AS3000 with 100 mlinjection volume Temperature: 25° C. Detectors: TSP UV 2000 UV/VISdetector at 254 nm Shodex differential refractometer RI 71 Evaluation:PSS-WinGPC Version 4.02

In Examples P1 to P3, a natural-rubber hot-melt pressure-sensitiveadhesive of overall formulation PA was prepared in each case in one ormore stages. All formulations are specified in phr, i.e. in relation to100 parts by mass of natural rubber.

The following components were used: Natural rubber SVR 5L (obtainablefrom Weber Schaer, Hamburg) Tackifier resin “Staybelite Resin”, ahydrogenated rosin (manufacturer: HERCULES) and HERCOTAC 205, anaromatic-modified aliphatic hydro- carbon resin (from Hercules BV,Rijswijk, NL) Filler Zinc oxide “Silox Actif” from the manufacturerSILOX, Belgium Phenolic an octylphenol-formaldehyde resin having amethylol crosslinking content of from 6 to 9% by weight (“Vulkaresen PA510” resin from the manufacturer HOECHST) Ageing Lowinox ® 22M46, a 2,2-inhibitor methylenebis[6-(1,1-dimethylethyl)- 4-methylphenol)] fromGREAT LAKES

In Examples P4 to P6, in addition to the formulation constituentsalready described, use is made of a β-pinene-based polyterpene resin(“Dercolyte S 115” from the manufacturer Lés Dérivés Résiniques &Térpeniques, Dax/France) and, as plasticizer, of the paraffinic whiteoil “Ondina G 17” from the manufacturer Deutsche SHELL AG, Hamburg);

In Example P4, in addition, a polychloroprene rubber from themanufacturer DU PONT (“Neoprene WRT”) was used;

In Example P5, in addition, a chlorinated copolymer of isobutylene andisoprene with the designation “EXXON® Chlorobutyl” from the manufacturerEXXON was used;

In Example P6, as halogenated rubber, in addition, a brominated butylrubber “Polysar Brombutyl X2” from the manufacturer BAYER, Leverkusenwas used; and

In Example P8 the reactive bromophenolic resin “SP 1056” (condensationproduct of octylphenol and formaldehyde having a methylol content offrom 9 to 13% by weight and a bromine content of 6-9% by weight), fromthe manufacturer SCHENECTADY EUROPE S. A., 62404 Béthune Cédex, France,was used.

The natural rubber was granulated, prior to use, in a granulator fromPallmann using small amounts of talc as release agent. The syntheticrubbers were also used in the form of granules.

The natural-rubber hot-melt PSAs prepared batchwise, as in Examples P1and P2, or continuously, as in Example P3, were applied, immediatelyafter their preparation, using a flexurally rigid 2-roll applicatorunit. The hot-melt PSA was applied in accordance with theabove-described 2-roll application process to a slightly creped paperbacking, impregnated in accordance with industry-standard techniques andequipped with release and primer layers, with an adhesive film thicknessof 50 μm. A coating nip was established between the first and thesecond, web-carrying coating roll in accordance with the applicationthickness. The first roll was temperature-controlled at 140° C., theweb-carrying roll at from 60 to 80° C. Depending on the particularexample, the natural-rubber hot-melt PSA supplied to the roll nip had atemperature of from 70° C. to 120° C.

Coating was carried out at the web speed adapted to the respectivepreparation process. In the case of continuous preparation of thehot-melt PSA, in Example P3, the natural-rubber hot-melt PSA wassupplied to the roll nip by means of a belt or conveying extruder.

Formulation PA phr Natural rubber SVR 5L 100.00 Staybelite Resin 35.29Hercotac 205 19.61 Vulkaresen PA 510 19.61 Silox actif 17.65 Lowinox ®22M46 3.92 Total 196.08

Example P1

In a first process step, a prebatch was prepared. The prebatch wasprepared in accordance with the formulation designated VB-PA in aBanbury kneading device of type GK 1.4 N from Werner & Pfleiderer,Stuttgart.

Prebatch Formulation VB-PA phr Natural rubber SVR 5L, granulated 100.00Resin, Hercotac 205 19.61 Lowinox 3.92 Silox actif 17.65

The kneading chamber and rotors were temperature-controlled at 25° C.and the blade speed was 50 min⁻¹. The overall weight of the prebatch was1.1 kg. All of the constituents were premixed in dry form and meteredtogether with the white oil. A mixing time of six minutes was sufficientto homogenize the constituents of the prebatch.

In process step 2, the natural-rubber hot-melt PSA was mixed tocompletion. For this purpose, all further additives were added to theprebatch in a kneading machine of type LUK1.0 K3 from Werner &Pfleiderer, Stuttgart, to give a natural-rubber hot-melt PSAcorresponding to the overall formulation PA. The prebatch was kneadedfor ½ a minute, then the entire tackifier resin including the reactivephenolic resin, in accordance with formulation F-PA, was added.

Formulation F-PA phr Prebatch VB-PA 141.18 Staybelite Resin 35.29Vulkaresen PA 510 19.61

The overall weight of the hot-melt PSA was 500 g. The chambertemperature was set at 80° C. throughout the process of mixing tocompletion. The overall kneading time was 10 minutes.

Example P2

Example P1 was repeated.

To simplify discharge, the second process step, that of mixing tocompletion, was carried out using a kneading machine of type VI U 20 Lfrom Aachener Misch-und Knetmaschinen-Fabrik Peter Küpper, Aachen, witha discharge screw. The prebatch was kneaded for ½ minute, then theentire tackifier resin and the reactive phenolic resin in accordancewith formulation F-PA were added.

The overall weight of the hot-melt PSA was 12 kg. The chambertemperature was set at 80° C. The overall kneading time was 10 minutes,the discharge time 7 minutes.

Example P3

The natural-rubber hot-melt PSA was prepared using a planetary rollextruder from ENTEX Rust & Mitschke with three roll cylinders. Thediameter of the roll cylinders was 70 mm. The first of the approachrings was provided with radial bores through which the liquids weresupplied by means of metering pumps. Gravimetric metering units,metering pumps, and the rotary speed of the central spindle were set soas to give a product rate of 65 kg/h with homogeneous mixing. Theindividual roll cylinders were temperature-controlled so as to give aproduct temperature of 80° C.

The adhesive tapes obtained in accordance with Examples P1 to P3crosslink following a 7-day storage period at 70° C. and are allsuitable as adhesive masking tapes with short-term temperaturestability >120° C. Results of Examples P1 to P3 Storage 4 days at roomStorage 7 Measurement temperature days at 70° C. method Shear test 20 Non PSTC 7 steel [min] EXAMPLE P1 23 4226 EXAMPLE P2 50 3850 EXAMPLE P312 2112 Bond strength on steel AFERA 4001 [N/cm] EXAMPLE P1 4.1 3.7EXAMPLE P2 4.1 3.5 EXAMPLE P3 4.5 4.0 Gel value Test method 3 EXAMPLE P14.3 30 EXAMPLE P2 2.7 29 EXAMPLE P3 5 19 Swelling test Test method 4EXAMPLE P1 0 3 EXAMPLE P2 0 3 EXAMPLE P3 0 2.5 Complex viscosity at Testmethod 6 T = 110° C., f = 0.1 rad/s [Pa · s] EXAMPLE P1 7.3 * 10⁴ 1.18 *10⁵ EXAMPLE P2 6.5 * 10⁴ 1.19 * 10⁵ EXAMPLE P3 5.1 * 10⁴ 1.01 * 10⁵

The crosslinking reaction is clearly evident from the measurements.

Examples P4 to P6 show hot-melt pressure-sensitive adhesive formulationsprepared with the process according to Example P1.

In the first process step, the formulation constituents natural rubber,synthetic rubbers, Silox actif, ageing inhibitor, and white oil wereeach mixed in an internal mixer with a mixing time of 4 minutes. Theother formulation constituents were metered in in the second processstep. Coating took place in each case in accordance with the 2-rollprocess described above. The hot-melt pressure-sensitive adhesives andthe coated adhesive tapes were tested after 4 days' storage time underambient climatic conditions and after 7 days of thermal conditioning at70° C.; all examples show unambiguous crosslinking.

The results are shown in table form.

Example P4

phr SVR 5L 78 Neoprene WRT 22 Staybelite Resin 46 Vulkaresen PA 510 6.0Dercolyte S 115 22 Silox actif 20 White oil 6 Total 200 Storage 4 daysat Storage 7 Measurement room temperature days at 70° C. method Sheartest 20 N on steel 32 180 PSTC 7 [min] Bond strength on steel 4.2 3.8AFERA 4001 [N/cm] Gel value 7.7 Not Test method 3 measured Swelling test0 1 Test method 4 Complex viscosity at 6.04E+04 9.01E+04 Test method 6 T= 110° C., f = 0.1 rad/s [Pa · s]

Example P5

phr SVR 5L 78 EXXON ® Chlorobutyl 22 Staybelite Resin 46 Vulkaresen PA510 6.0 Dercolyte S 115 22 Silox actif 20 White oil 6 Total 200 Storage4 days at Storage 7 Measurement room temperature days at 70° C. methodShear test 20 N on steel 38 629 PSTC 7 [min] Bond strength on steel 4.13.5 AFERA 4001 [N/cm] Gel value 7.4 Not Test method 3 measured Swellingtest 0 1 Test method 4 Complex viscosity at 6.53E+04 9.37E+04 Testmethod 6 T = 110° C., f = 0.1 rad/s [Pa · s]

Example P6

phr SVR 5L 78 Polysar Brombutyl X2 22 Staybelite Resin 46 Vulkaresen PA510 6.0 Dercolyte S 115 22 Silox actif 20 White oil 6 Total 200 Storage4 days at Storage 7 Measurement room temperature days at 70° C. methodShear test 20 N on steel 25 7406 PSTC 7 [min] Bond strength on steel 2.92.7 AFERA 4001 [N/cm] Gel value 9.1 Not Test method 3 measured Swellingtest 0 4 Test method 4 Complex viscosity at 7.79E+04 1.37E+05 Testmethod 6 T = 110° C., f = 0.1 rad/s [Pa · s]

Example P7

phr SVR 5L 100.00 Staybelite Resin 38 Hercotac 205 11 Vulkaresen PA 51019 Silox actif 17 Sontal 3.8 Storage 4 days at Storage 7 Measurementroom temperature days at 70° C. method Shear test 20 N on steel103 >10,000 PSTC 7 [min] Bond strength on steel 2.9 2.5 AFERA 4001[N/cm] Gel value 7.1 Not Test method 3 measured Swelling test 0 6 Testmethod 4 Complex viscosity at 1.60E+05 5.42E+05 Test method 6 T = 110°C., f = 0.1 rad/s [Pa · s]

Example P8

phr SVR 5L 100 Staybelite Resin 19 Hercotac 205 37 Harz SP 1056 11 Siloxactif 19 Storage 4 days at Storage 7 Measurement room temperature daysat 70° C. method Shear test 20 N on steel 324 >10,000 PSTC 7 [min] Bondstrength on steel 2.5 2.4 AFERA 4001 [N/cm] Gel value 5.3 Not Testmethod 3 measured Swelling test 1 6 Test method 4 Complex viscosity at1.60E+05 3.42E+05 Test method 6 T = 110° C., f = 0.1 rad/s [Pa · s]

Example P9

A masking tape was produced in accordance with the process from ExampleP3 and investigated in accordance with Test Method 5. The drop in theloss angle tan δ at temperatures above 130° C. shows the crosslinkingprocess of the hot-melt pressure-sensitive adhesive on the paper backingduring the measurement procedure (see FIG. 1). Phr SVR 5L 100 StaybeliteResin 50 Vulkaresen PA 510 10 Silox actif 33 White oil 15 Dercolyte S115 42

1. Hot-melt pressure-sensitive adhesive based one or more non-thermoplastic elastomers, comprising at least 100 parts by mass of one or more non-thermoplastic elastomers, from 1 to 200 parts by mass of one or more tackifying resins, from 1 to 100 parts by mass of one or more reactive phenolic resins whose methylol content is from 1 to 20% by weight based on the reactive phenolic resin, and from 1 to 100 parts by mass of accelerator substances.
 2. Hot-melt pressure-sensitive adhesive according to claim 1, wherein the non-thermoplastic elastomers are selected from the group consisting of natural rubbers, random-copolymerized styrene-butadiene rubbers (SBR), butadiene rubbers (BR), synthetic polyisoprenes (IR), butyl rubbers (IIR) and ethylene-vinyl acetate copolymers (EVA).
 3. Hot-melt pressure-sensitive adhesive according to claim 1, based on a polymer blend of one or more of the non-thermoplastic elastomers and one or more thermoplastic elastomers selected from the group consisting of polypropylenes, polyethylenes, metallocene-catalysed polyolefins, polyesters, polystyrenes and synthetic block copolymer rubbers.
 4. Hot-melt pressure-sensitive adhesive according to claim 1, wherein the crosslinking accelerator substances are selected from the group consisting of chloroprenes, chlorinated butyl rubbers, brominated butyl rubbers, chlorosulphonated polyethylenes, metal oxides, organic acids or salts thereof, metal stearates and metal resinates.
 5. Hot-melt pressure-sensitive adhesive according to claim 1, wherein the reactive phenolic resins are halogenated and have a halogen content of from 1 to 20% by weight, based on the reactive phenolic resin.
 6. Hot-melt pressure-sensitive adhesive according to claim 1, wherein the reactive phenolic resin comprises a mixture of different reactive phenolic resins having different reactivities.
 7. Hot-melt pressure-sensitive adhesive according to claim 1, wherein fillers are added to the adhesive which are selected from the group consisting of metal oxides, chalks, precipitated or pyrogenic silicas, solid or hollow glass beads, microballoons, carbon blacks, glass fibres, polymer fibres and combinations thereof.
 8. Hot-melt pressure-sensitive adhesive according to claim 1, wherein plasticizers are added to the adhesive which are selected from the group consisting of paraffinic or naphthenic oils, oligomeric nitrile rubbers, liquid isoprene rubbers, oligobutadienes, soft resins, wool fats, rapeseed oils, castor oils and combinations thereof.
 9. Self-adhesive article comprising the hot-melt pressure-sensitive adhesive of claim 1 applied to at least one side of a web-form material.
 10. Self-adhesive article according to claim 9, wherein the thickness of the hot-melt pressure-sensitive adhesive on the web-form material is between 5 μm and 3000 μm.
 11. Self-adhesive article according to claim 10, wherein the hot-melt sensitive adhesive is applied in a thickness of from 40 μm to 1500 μm to a release paper having an anti-adhesive coating on both sides.
 12. Process for producing self-adhesive articles wherein the hot-melt pressure-sensitive adhesive of claim 1 is applied to a web form material with the aid of a multi-roll applicator unit which comprises from two to five rolls. 